The overall aim of this proposal is to understand the mechanism(s) of gene amplification and chromosome rearrangements as studied with methotrexate resistance and the dihydrofolate reductase gene. We believe these processes involve two steps: 1) overreplication of DNA, 2) recombination to generate amplified genes as well as a wide variety of chromosomal aberrations- rearrangements. Our specific aims include: A: To define the molecular events that occur during the time that DNA replication is inhibited, which upon resumption result in overreplication of DNA. B: To determine the temporal relationship between transcription and replication of the dihydrofolate reductase gene. C: To determine if actively transcribed genes are more prone to amplification and constitute "hot spots" for recombination events. D: To determine if the time of replication of a gene (or DNA sequence) is related to its propensity for amplification.